1. Field of the Invention
The present invention relates to an electroluminescent display device (ELD), and more particularly, to an electroluminescent display having a dual-plate structure in which a thin film transistor array and an organic electroluminescent part are formed in separate substrates.
2. Discussion of the Related Art
Recently, flat displays are vigorously developed. Among the developed flat displays, liquid crystal displays (LCD), field emission displays (FED), electroluminescent devices (ELD), and plasma display panels (PDP) are spotlighted.
Among the flat displays, the LCD is widely used in personal information devices such as personal communication service terminals, laptop computers, TVs, etc. However, since the LCD has a narrow viewing angle and a longer response time, a self-luminescent organic electroluminescent display device is spotlighted.
The organic electroluminescent display device uses electroluminescence phenomena that an electric field is applied to a negative electrode and a positive electrode formed in the upper and lower sides of an organic emitting layer such that electrons and holes are inserted and transported into the organic emitting layer so that when the electrons and holes are recombined, the energy will be released in the form of a visible light. In other words, the electrons and the holes combine as hole-electron pairs and the hole-electron pairs emit light when returning from the excited state to the ground state.
Due to a faster response time, excellent brightness, and thin films, the organic electroluminescent display device can be driven at low voltage and display all colors within the visible spectrum. Therefore, the organic electroluminescent display device can satisfy modern people's tastes. Moreover, the organic electroluminescent display device can be implemented on a flexible transparent substrate such as a plastic substrate.
In addition, since the organic electroluminescent display device can be driven under a low voltage, consumes relatively low power, and easily displays three colors, i.e., green, red, and blue colors, the organic electroluminescent display device is ideal as a next generation flat display.
The organic electroluminescent display device can be divided into a passive matrix type and an active matrix type based upon the driving method thereof.
The passive matrix type organic electroluminescent display device has a simple structure and requires a simple manufacturing method. However, the passive matrix type organic electroluminescent display device has high power consumption and it is difficult to achieve a large-size organic electroluminescent display device. Moreover, the greater the number of lines, the lower the aperture ratio.
On the other hand, the active matrix type organic electroluminescent display device has high luminescent efficiency and provides high-density resolution.
Hereinafter, the conventional active matrix type organic electroluminescent display device, particularly, the conventional active matrix type organic electroluminescent display device having the dual-plate structure will be described in detail with reference to the accompanying drawings.
FIG. 1 illustrates a cross-sectional view of a conventional organic electroluminescent display device. FIG. 2 illustrates a plan view of the conventional organic electroluminescent display device.
As shown in FIG. 1, the conventional organic electroluminescent display device is fabricated by bonding a first transparent substrate 10 to a second transparent substrate using a sealant. A plurality of pixels (luminescent parts) P is defined on the first substrate 10, and thin film transistors T and array lines (not shown) are formed on the respective sides of the respective pixels P.
In other words, on the first substrate 10, the gate lines arranged in a single row, the data lines and power lines crossing each other and spaced apart from each other by a predetermined interval, the switching thin film transistors Ts (not shown) provided at locations where the gate lines cross the data lines, and the driving thin film transistors Tp provided at locations where the gate lines cross the power lines, are formed. The drain electrodes of the switching thin film transistors are connected to the gate electrodes 12a of the driving thin film transistors and the drain electrodes 15b of the driving thin film transistors are integrally formed with the connection electrodes 41.
Each set of the gate lines, the data lines and the power lines define a pixel region. The corresponding switching thin film transistor Ts and driving thin film transistors Tp are provided in the pixel region.
First electrodes 21 as transparent hole injection electrodes are formed on the upper side of the second substrate 20. Organic common layers 22 consisting of a hole injection layer and a hole transporting layer are formed on the upper side of the first electrodes 21. Organic luminescent layers 23 are formed on the organic common layers 22. Electron injection layers 25 are formed on the organic luminescent layers 23. Second electrodes 24 as electron injection electrodes are formed on the electron injection layers 25.
The second electrodes 24 receive the electric signals from the peripheral region surrounding the active region on which images are displayed, and the first electrodes 21 receive electric signals through the connection electrodes 41 in the contact parts C.
Each of the connection electrodes 41 is formed on the first substrate 10, and contacts the second electrode 24 when bonding the first substrate 10 to the second substrate 20 such that the signals are applied to the first and second electrodes.
The connection electrodes 41 contact the second electrodes 24 and cover the contact spacers 42. The contact spacers 42 have a column-shape with a predetermined height and are formed between the two substrates to maintain a cell gap.
The organic luminescent layers 23 express colors of red R, green G, and blue B, and separate organic materials are patterned in the respective pixels.
When an electric field is applied to the first electrodes 21 and the second electrodes 24 of the organic electroluminescent display device, electrons are injected into the organic luminescent layer 23 from the second electrodes 24, and holes are injected into the organic luminescent layer 23 from the first electrodes 21.
The electrons and the holes injected into the organic luminescent layer 23 move to the organic luminescent layer 23 under the electric field and are recombined with each other to form excitons. The electrons in the excited state of the excitons return to the ground state and emit a visible light.
Meanwhile, the patterning of the first electrodes, the second electrodes, the organic luminescent layers, electron injection layers and the organic common electrodes provided in the second substrate 20 will be described with reference to FIGS. 1 and 2.
First, a transparent conductive metal is deposited on the second substrate 20 to form the first electrodes 21 as anodes in the form of lines or pixels. Subsequently, an organic material such as photoacryl, polyimide, or the like is coated on the anode. Then the separators 32 are formed by performing photo-etching, thereby defining the contact part C surrounded by the separators 32.
Next, on the upper side of the first electrodes 21 between the separators 32, the R-, G-, and B-organic common layer 22, the R-, G-, and B-organic luminescent layers 23, and the R-, G-, and B-electron injection layers 25 are formed. The R-, G-, and B-organic common layer 22, the R-, G-, and B-organic luminescent layers 23, and the R-, G-, and B-electron injection layers 25 are respectively formed using a shadow mask 170.
More specifically, the shadow mask is disposed such that only the regions corresponding to the pixels of green color G are opened, and a G-organic common layer 22G, a G-organic luminescent layer 23G and a G-electron injection layer 25G are deposited in turn. Then the shadow mask is shifted to open only the regions corresponding to the pixels of blue color B, a B-organic common layer 22B, a B-organic luminescent layer 23B and a B-electron injection layer 25B are deposited in turn. Subsequently, the shadow mask is shifted to open only the regions corresponding to the pixels of red color R, and an R-organic common layer 22R, an R-organic luminescent layer 23R and an R-electron injection layer 25R are deposited in turn. During this process, the contact part is covered with the shadow mask such that the organic common layer, the organic luminescent layers and the electron injection layers are not formed in the contact part.
Further, the second electrodes 24 are formed on the second substrate 20 using an opened mask. Therefore, the second electrodes 24 in the contact part C are formed on the first electrodes 21 to electrically contact with each other. In addition, the second electrodes 24 in the regions other than the contact part C are formed on the electron injection layers 25. In the contact parts C, the first electrodes 21 connect the connection electrodes 41 indirectly via the second electrodes 24 to transmit electric signals.
If using the opened mask to form the organic common layers, the organic luminescent layers and the electron injection layers 25, insulator organic common layers, insulator organic luminescent layers and electron injection layers will be formed in the contact part so that the first electrodes are not electrically connected to the second electrodes. In order to prevent this, the shadow mask is used when forming the organic common layers, the organic luminescent layers and the electron injection layers. However, when using the shadow mask to form the organic common layers, the organic luminescent layers and the electron injection layers, the process time is prolonged and is complicated.